Abstract: To clarify the impact of the Miller cycle on the performance evolution of diesel engines under varying altitude conditions and to address the operational demands of hybrid-specific diesel engines in high-altitude environments， a one-dimensional simulation study was conducted based on a turbocharged diesel engine. The research systematically examined the effects of different altitude levels， Miller-cycle strategies （early intake valve closing（EIVC） and late intake valve closing（LIVC）） on volumetric efficiency， thermal efficiency， power， fuel consumption and NO_x emissions at different engine speeds. The results show that under plain conditions， a slight advance in intake valve closing achieves the optimal compromise between torque and fuel consumption. Between the two Miller-cycle strategies， EIVC favors air charging at 1 600 r/min， while LIVC is more beneficial at 2 400 r/min. Elevating altitude from 3 km to 4 km leads to a notable decline in engine performance. Volumetric efficiency， thermal efficiency， and power decrease by 18.05%， 18.76% and 19.56%， respectively， with a concurrent increase in fuel consumption of 24.24%. NO_x emissions exhibit a ring-shaped distribution， with the peak emission zone located around 2 km altitude and Miller-cycle ranges of the intake valve closing advance angle 40°，30°，20°，10°， the original intake value closing angle， and the intake value closing angle delay 10°，20° and 30°. Deviating from the central region， whether by altering altitudes or Miller intensities， can effectively reduce NO_x emissions， albeit through distinct mechanisms. When applying deeper Miller-cycle strategies （either EIVC or LIVC） to lower NO_x emissions， a balance between dynamic performance and fuel economy must be considered.